1. Field of the Invention
The present invention relates to a method of correcting the light amount of a printhead where plural light-emitting chips, in which plural light-emitting elements are disposed in a row, are disposed in a row.
2. Description of the Related Art
Conventionally, in an electrophotographic printer, a photosensitive drum whose surface has been uniformly and evenly charged is exposed to form an electrostatic latent image, the electrostatic latent image is developed into a toner image, and the toner image is transferred and fixed to a printed medium. Incidentally, in a case where the photosensitive drum is exposed, an electrophotographic printer that uses an LED printhead using LEDS as light-emitting elements has been proposed because the photosensitive drum can be activated at a high speed and is compact.
Here, description will be given of an LED printhead 7 using FIG. 6. As shown in the drawing, plural LED elements 3 are arranged in a row on LED chips 1. The LED chips 1 are disposed in a row on a substrate 5.
The pitch at which the LED elements 3 are disposed is set to a value that is the same as the resolution pitch at which the LED printhead is disposed.
However, in this LED printhead 7, whereas the pitch (P) of the LED elements 3 inside the LED chips 1 can be manufactured with high precision, the precision of the pitch (P′) of LED elements 3′ at joints 9 of the LED chips 1 is poor due to problems in terms of facilities. For example, whereas the tolerance of the pitch (P) of the LED chips 3 inside the LED chips 1 is ±1 μm, the tolerance of the pitch (P′) of the LED elements 3′ at the joints 9 of the LED chips 1 is ±10 μm. Thus, in many cases, P is less than P′, and there are many cases where white stripes occur.
Thus, it has been proposed to prevent the occurrence of white stripes by making the pitch (P) of the LED elements 3 inside the LED chips 1 narrower than the pitch P′ of the LED elements 3′ at the joints 9 of the LED chips 1.
However, this proposal does nothing more than lower the frequency with which white stripes occur and cannot completely eliminate the occurrence of white stripes.
Thus, it has been proposed to raise the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 by uniformly increasing the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be 2 to 6% more than the current supplied to the other LED elements 3.
It has also been proposed to increase the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be more than the current supplied to the other LED elements 3 as long as the pitch of the joints of the LED chips is equal to or greater than a predetermined value. For example, it has been proposed to increase the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be 2% more than the current supplied to the other LED elements 3 and to raise the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 in a case where the pitch (P′) of the LED elements 3′ at the joints 9 of the LED chips 1 is 66 μm or greater and less than 69 μm, and to increase the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be 4% more than the current supplied to the other LED elements 3 and to raise the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 in a case where the pitch (P′) is 69 μm or greater (e.g., see JP-A-2001-80111).
However, there are the following problems in the method where the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 is raised by uniformly increasing the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be more than the current supplied to the other LED elements 3.
Because the tolerance of the pitch (P′) of the LED elements 3′ at the joints 9 of the LED chips 1 is ±10 μm whereas the tolerance of the pitch (P) of the LED elements 3 inside the LED chips 1 is ±1 μm, it is not always the case that white stripes occur but there are also cases where black stripes occur. Thus, when the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 is raised by increasing the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be more than the current supplied to the other LED elements 3, there are also cases where the black stripes are accentuated.
Also in the method of raising the light amount of the LED elements 3′ at the joints 9 of the LED chips 1 by uniformly increasing the current supplied to the LED elements 3′ at the joints 9 of the LED chips 1 to be more than the current supplied to the other LED elements 3 as long as the pitch of the joints of the LED chips 1 is equal to or greater than a predetermined value, consideration is only given to the pitch of the joints of the LED chips 1 and not to the profiles of the beams emitted from the LED elements 3. Thus, in a case where the beam profiles of the LED elements 3′ at the joints 9 of the LED chips 1 greatly differ from those of the other LED elements, there are the problems that correction cannot be conducted well and white stripes and black stripes occur. Also, because the current supplied to the LED elements 3′ is only altered at the two stages of 2% and 4%, the correction resolution is great. Thus, there is also the problem that white stripes and black stripes occur depending on the printing pattern.